1. Field of the Invention
This invention relates to a method of implanting ions into a substrate wafer and an ion implantation apparatus for implanting ions into a substrate wafer. Example applications of the ion implantation method and apparatus include the separation or exfoliation of thin layers of crystalline semiconductor material, such as silicon, in the production of photovoltaic cells and in the production of Silicon on Insulator (SOI) wafers.
2. Background Information
Methods for exfoliation of thin layers of silicon have been developed which typically involve ion implantation of hydrogen and/or helium into a crystalline silicon substrate followed by annealing, under process conditions which facilitate substantially uniform shearing. For example, high implant doses, such as 7e16 atoms/cm2 of hydrogen in the case of hydrogen implantation, are used. The implantation at high doses causes damage to the silicon crystal and with the post implantation annealing, the implanted layer creates internal pressure in the form of bubbles resulting in the fracture of the silicon crystal and exfoliation of the overlying silicon layer.
During implantation, damage to the silicon crystal is created in the form of lattice dislocations. Some damage at the end of range is required for the exfoliation process. However, in order to optimize performance in the resulting SOI or photovoltaic device, damage to the bulk silicon should be mitigated. Post implantation annealing is performed to reduce lattice dislocations. It is also known that implantation at higher temperatures reduces damage to the crystal during the implantation process. However, during implantation, the temperature of the silicon substrate is normally controlled to prevent premature exfoliation which may occur at temperatures of typically greater than 250° C.
Batch type ion implantation apparatus is known which allows for a batch of wafers to be processed simultaneously by mounting the wafers round the periphery of a process wheel mounted for rotation about an axis, so that the wafers on the wheel pass one after the other through a beam containing the ions to be implanted. The wafers are mounted on cooled wafer carriers. The wafer carriers provide wafer support surfaces canted inwards slightly, so that on rotation of the process or implant wheel, wafers are pressed by centrifugal force against the wafer support surfaces. Fences are located at edges of the wafer support surfaces which are outermost relative to the axis of rotation of the process wheel, to prevent wafers from sliding outwards on the wafer carriers. Such an apparatus thus enables a high current ion beam to be used which increases throughput whilst maintaining the temperature of the wafers within an acceptable range. Thus, there are known arrangements for cooling the wafers to an acceptable temperature range, but there remains a need to control the temperature of the silicon substrate during ion implantation to improve the process. There is also a need to avoid excessive temperature variation over the wafer area during implantation.